Various filters for clarifying a fluid are presently developed and produced. Among them, cartridge-type filters (hereinafter called filter cartridges) are widely used in the industrial field, for example, for removing suspended particles in industrial liquid materials, removing cakes flowing out of a cake filtering apparatus and clarifying industrial water.
Several kinds of structures of a filter cartridge have so far been proposed. The most typical one is a bobbin winder-type filter cartridge, which is a cylindrical filter cartridge prepared by winding a spun yarn as a filter material on a perforated cylindrical core in a twill form and then fluffing the spun yarn. This type has long been used due to inexpensiveness and easiness in production. Another type of structure includes a non-woven fabric-laminated type filter cartridge. This is a cylindrical filter cartridge prepared by winding several kinds of non-woven fabrics such as a carding non-woven fabric stepwise and concentrically on a perforated cylindrical core. A recent advanced technique in a non-woven fabric production has allowed some of them to be put to practical use.
However, the above-mentioned filter cartridges have several defects. For example, in the bobbin winder-type filter cartridge for trapping foreign matters by means of fluffs of fluffed spun yarns and also in gaps of the spun yarns, it is difficult to control the size and form of the fluffs and gaps. This limits size and amount of the foreign matters that can be trapped. Further, constitutional fibers of a spun yarn, which is made from short fibers, fall away when fluid flows onto the filter cartridge. Furthermore, in producing a spun yarn, a trace amount of a surfactant is often applied onto a surface of material short fibers to prevent the short fibers from sticking to a spinning machine by electrostatic charge or the like. Filtering a liquid by means of a filter cartridge using surfactant-coated spun yarns may bring adverse effects on the cleanness of liquid, such as foaming of the liquid, and increase in TOC (total organic carbon), COD (chemical oxygen demand) and the electric conductivity. In addition, a spun yarn is produced by spinning short fibers as already mentioned, for which at least two steps of forming and spinning short fibers are required. Thus, use of the spun yarn will sometimes increase a price of the product.
In a filter in which a broad non-woven fabric is wound around a perforated cylinder in layers as shown in FIG. 1, a so-called non-woven fabric-laminated type filter cartridge, its performance depends on the non-woven fabric used. A non-woven fabric is produced mostly by a method in which short fibers are confounded by means of a carding machine or an air laid machine and then subjecting them, if necessary, to heat treatment by means of a hot-air heater or a heating roll, or a method in which a non-woven fabric is directly prepared, such as a melt blowing method and a spun bonding method. However, any machines used for producing non-woven fabrics, such as a carding machine, an air laid machine, a hot-air heater, a heating roll, a melt blowing machine and a spun bonding machine, may cause, for example, uneven basis weights of a non-woven fabric in a lateral direction of a machine. Accordingly, a filter cartridge of poor quality will be produced. Also, use of a more advanced manufacturing technique to avoid such unevenness sometimes raises the production cost. Moreover, production of one kind of non-woven fabric-laminated type filter cartridges needs two to six kinds of non-woven fabrics, and different non-woven fabrics are needed depending on the kind of a filter cartridge. Thus, the production cost will increase in some cases.
Several methods have been proposed in order to solve such problems of conventional filter cartridges.
For example, Japanese Patent Publication No. 15004/1988 (U.S. Pat. No. 4,278,551) proposes a porous winding cartridge filter comprising a tubular member formed from a superimposed winding body of a continuous yarn bundle, whose surface has been modified with cationic colloidal silica. According to this gazette, the filter has a higher foreign matter-removing rate than that of conventional bobbin winder filters due to the cationic silica colloid. However, use of cationic silica colloid is considered to affect cleanness of a liquid as described above.
Further, Japanese Utility Model Publication No. 7767/1994 proposes a filter cartridge in which a filter material obtained by squashing a tape-shaped paper having porosity while twisting, thereby squeezing it to control a diameter thereof to about 3 mm is wound around a porous internal cylinder in a close twill. This method is advantageous in that a winding pitch can be gradually increased from the porous internal cylinder toward the outside. However, the filter material needs to be squashed and squeezed, so that foreign matters are trapped primarily between the winding pitches of the filter material. Accordingly, it is less expected to trap foreign matters by the filter material itself as is the case of a conventional bobbin winder type filter using spun yarns which traps foreign matters by means of fluffs. This blocks the surface of the filter to shorten the filter life or brings about the poor liquid-passing property in a certain case. Japanese Patent Publication No. 25607/1989, Japanese Utility Model Publication No. 52090/1991 and Japanese Patent Application Laid-Open No. 317513/1989 concern the invention analogous to the aforementioned publication, and all these publications involve the similar problems.
Alternatively, Japanese Patent Application Laid-Open No. 115423/1989 proposes a filter in which strings obtained by slitting a cellulose spun bonded non-woven fabric into strips and passing them through narrow holes to twist them are wound around a bobbin having a lot of drilled pores. It is considered that this method shall make it possible to prepare a filter having a higher mechanical strength and being free of dissolution in water and elution of a binder, as compared with a conventional roll tissue filter prepared by winding tissue paper in a roll form, which is produced from α-cellulose prepared by refining a coniferous pulp. However, the cellulose spun bonded non-woven fabric used for this filter has a papery form and thus a too high rigidity, so that it is less expected to trap foreign matters by the filter material itself as is the case of a conventional bobbin winder type filter using spun yarns which traps foreign matters by means of fluffs. Further, the cellulose spun bonded non-woven fabric is liable to swell in a liquid due to its papery form. Swelling may bring about various problems such as a decrease in a filter strength, a change in a filtering accuracy, a deterioration in a liquid-passing property, a reduction in a filter life and the like. Adhesion at fiber intersections of the cellulose spun bonded non-woven fabric are mostly conducted by a certain chemical treatment. Such adhesion is often unsatisfactory, causing a change in a filtering accuracy or falling of fiber chips, so that a stable filtering performance is difficult to achieve. Other inventors propose in Japanese Utility Model Application Laid-Open No. 36878/1979 a filter using a tape-shaped cellulose non-woven fabric without using a binder, but the filter has the same problem.
Further, Japanese Patent Application Laid-Open No. 45810/1992 proposes a filter prepared by winding a slit non-woven fabric comprising composite fibers in which 10% by weight or more of structural fibers is divided ones of 0.5 denier or less on a porous core cylinder to provide the fiber density of 0.18 to 0.30. This method is advantageously used to trap fine particles contained in a liquid by means of fibers having a small fineness. However, in order to divide the composite fibers, a stress needs to be applied using, for example, high-pressure water, and it is difficult to evenly divide the fibers all over the non-woven fabric by means of high-pressure water processing. If not evenly divided, there occurs a difference in a scavenged particle diameter between a well-divided portion and an insufficiently divided portion of the non-woven fabric, and this may roughen the filtering accuracy. Further, the stress applied for dividing sometimes lowers a strength of the non-woven fabric, and this may cause reduction of the resulting filter strength and frequent deformation of the filter during use; or possible change of the void ratio of the filter may reduce the liquid-passing property. Further, the reduced strength of the non-woven fabric makes it difficult to control a tension in winding around a porous core cylinder, and hence the difficulty in exact control of the void rate may arise. Further, a spinning technique required for producing easily divisible fibers and an increased operation cost in producing thereof lead to an increased production cost of the filter. Such a filter would be usable in a certain field such as the pharmaceutical industry and the electronic industry which require a high filtering performance, if the above mentioned problems of the filtering performance are solved. However, such a filter is considered to be difficult to use in cases in which inexpensive filters are requested such as the filtering of swimming pool water and a plating liquid for the plating industry. Analogous inventions include Japanese Patent Application Laid-Open No. 45811/1992, Japanese Utility Model Application Laid-Open No. 131412/1992, Japanese Utility Model Application Laid-Open No. 131413/1992, Japanese Utility Model Application Laid-Open No. 2715/1993 and Japanese Utility Model Application Laid-Open No. 18614/1993, all of which involve the problems described above.
Japanese Patent Application Laid-Open No. 60034/1995 proposes a filter prepared by winding a non-twisted, flat tape-shaped fiber around a porous core cylinder, the tape-shaped fiber being prepared by sterically crimping an eccentric sheath-core type of combined short fibers comprising two components with different heat shrinkability. According to this gazette, the filter has less bubbling and less discharged fiber chips than those of conventional filters. However, fibers constituting this filter have no adhesion between yarns, though they have a steric crimping property. Because of this, trapped foreign matters may easily move into the filtrate when a filtering pressure is raised. Japanese Patent Application Laid-Open No. 328356/1995, analogous to the above application, also involves the problem described above.
An object of the present invention is to solve the problems described above. It has been found, as a result of investigations, that a cylindrical filter cartridge which is excellent in a liquid-passing property, a filter life and a stability of a filtering accuracy can be obtained by winding a long fiber non-woven fabric comprising thermoplastic fibers on a perforated cylinder in a twill form. This finding has led to the present invention.